352-001 · Question #768
A large enterprise company is redesigning their WAN infrastructure and has these design requirements: - The Frame Relay WAN infrastructure will be replaced with an MPLS VPN service. - BGP is the…
The correct answer is B. Summarize the routes on the MPLS WAN interfaces of the distribution routers. D. Configure the distribution routers to send a default route to the MPLS network. Because the SP VRF supports only 1000 prefixes but the enterprise has 3000 routes, summarizing routes reduces the advertised prefix count while sending a default route from distribution routers ensures complete reachability for access routers.
Question
A large enterprise company is redesigning their WAN infrastructure and has these design requirements:
- The Frame Relay WAN infrastructure will be replaced with an MPLS VPN
service.
- BGP is the PE-CE routing protocol adopted for this scenario.
- There are 3000 routes in the routing database of the distribution
routers.
- All 3000 routers will be advertised from the access routers through
the MPLS network.
- The service provider that provides this MPLS VPN service supports
only 1000 prefixes per VRF. Which two design solutions ensure that the enterprise access routers reach all devices in this enterprise network? (Choose two)
Options
- AConfigure the access routers to send a default route to the MPLS network.
- BSummarize the routes on the MPLS WAN interfaces of the distribution routers.
- CUse prefix lists on the distribution routers to control which routes are sent to the MPLS network.
- DConfigure the distribution routers to send a default route to the MPLS network.
- EConfigure null routes and aggregate routes for the prefixes in your network on the distribution
How the community answered
(32 responses)- A9% (3)
- B66% (21)
- C6% (2)
- E19% (6)
Why each option
Because the SP VRF supports only 1000 prefixes but the enterprise has 3000 routes, summarizing routes reduces the advertised prefix count while sending a default route from distribution routers ensures complete reachability for access routers.
Configuring access routers to send a default route into the MPLS network advertises a default outbound from spoke sites, which does not help access routers receive reachability information for the 3000 enterprise destinations.
Summarizing the 3000 enterprise routes into aggregate prefixes on the distribution router WAN interfaces reduces the number of prefixes advertised into the MPLS VPN, keeping the count within the SP's 1000-prefix-per-VRF limit while maintaining reachability to all destinations.
Applying prefix lists to filter routes would selectively block some prefixes from being advertised, causing those destinations to become unreachable rather than ensuring full connectivity.
Advertising a default route from the distribution routers into the MPLS network ensures access routers have a forwarding path to all enterprise destinations even when specific prefixes are summarized, guaranteeing full end-to-end connectivity without exceeding the SP prefix limit.
Null routes are used locally to prevent routing loops when summarizing but do not by themselves ensure that all 3000 destinations remain reachable across the MPLS VPN.
Concept tested: MPLS VPN BGP PE-CE route scaling with summarization and default route
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/xe-16/mp-l3-vpns-xe-16-book/mp-bgp-mpls-vpn.html
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